In 1996, more than 75 million people worldwide used cellular telephones. Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. Within the United States, cellular service is offered not only by dedicated cellular service providers, but also by the regional Bell companies, such as U.S. West, Bell Atlantic and Southwestern Bell, and the national long distance companies, such as AT&T and Sprint. The enhanced competition has driven the price of cellular service down to the point where it is affordable to a large segment of the population.
This competition has also led to a rapid and sweeping innovation in cellular telephone technology. Analog cellular systems are now competing with digital cellular systems. Older frequency division multiple access (FDMA) and time division multiple access (TDMA) systems are now competing with code division multiple access (CDMA) systems. In order to maximize the number of subscribers that can be serviced in a single cellular system, frequency reuse is maximized by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. Accordingly, the increased number of cellular base stations has resulted in increased infrastructure costs. To offset this increased cost, cellular service providers are eager to implement any innovations that may reduce equipment costs, maintenance/repair costs, and operating costs, or that may increase service quality/reliability, and the number of subscribers that the cellular system can service.
Much of this innovation has focused on service quality improvements, such as expanded digital PCS services or smaller and lighter cellular phone handsets having a longer battery life, or equipment cost reduction, such as smaller, cheaper, more reliable transceivers for the cellular base stations. However, there has been only limited innovation related to the reducing the operating costs of a cellular system. Electrical power is one of the more significant operating costs of a cellular system. Every cellular base station has a transmitter for sending voice and data signals to mobile units (i.e., cell phones, portable computer equipped with cellular modems, and the like) and a receiver for receiving voice and data signals from the mobile units. The transmitter and receiver both use power amplifiers to increase the strengths of received signals and transmitted signals.
In the prior art CDMA cellular systems, power control has been implemented to control the power transmitted by a nearby mobile unit and the power transmitted by a distant mobile unit. This is done to overcome the near-far effect, whereby the stronger signal from a nearby mobile unit overwhelms the weaker signal of a distant mobile unit. The power output level of the mobile units is adjusted according to a combination of the received signal strength in the mobile unit and fine adjustment messages received from the base transceiver station.
CDMA systems use some coded channels as control channels. The control channels include a pilot channel, a synchronization channel, a paging channel and an access channel. After a mobile unit is powered up, the mobile unit sends out a registration message and then monitors the pilot, paging, and synchronization channels in order to establish a communication link with a base transceiver station.
The pilot signal is transmitted at constant power by the base transceiver station. A newly activated mobile unit sees a stronger or weaker pilot signal, depending on where the mobile unit is located with respect to the base transceiver station. The mobile unit tries to access the base transceiver station at a first selected transmitted signal power level. If the access fails, the mobile unit tries again at a higher transmitted signal power level. This process continues until the mobile unit either times out or accesses the base transceiver station.
Once the mobile unit accesses the base transceiver station, the mobile unit begins transmitting voice and/or data signals at a nominal transmitted signal power level established by the base transceiver station. Thereafter, the base transceiver station sends signals containing an UP/DOWN bit to the mobile units that cause the mobile units to increase power (i.e., UP) by small amount if the UP/DOWN bit is set, or reduce power (i.e., DOWN) by a small amount if the UP/DOWN bit is not set. The power of a nearby mobile unit is reduced, and the power of a distant mobile unit is increased, by the separate UP/DOWN control signals repetitively sent to each by the base transceiver station. The process is repeated in multiple message frames until the two signals are received at the base transceiver station at roughly equal power levels.
CDMA systems also implement power control over the RF signal transmitted by the base transceiver station. This type of power control is implemented to minimize interference with transmissions in adjacent cells. In some CDMA systems, the base transceiver station automatically steps down the RF output power level in the traffic channels by a specified decrement, .DELTA.1. The value of .DELTA.1 is sufficiently small so that the output power level of the RF signal transmitted by the base transceiver station gradually reduces over a number of message frames. However, once a mobile unit determines that the signal from the base transceiver station is unacceptably weak, for example, by detecting five bad frames in a row, the mobile unit transmits a power control signal to the base transceiver station that causes the base transceiver station to begin to step up the power level by a specified increment, .DELTA.2.
However, the prior art CDMA cellular systems do not exercise power control within the base station itself. CDMA systems cannot tolerate large amounts of signal distortion, and therefore require the use of RF amplifiers having good linearity characteristics across a wide range of operating conditions in order not to violate the IS 95 bandwidth requirements due to spectral spreading effects. Unfortunately, the DC-to-RF conversion efficiency for linear RF amplifiers is very low. CDMA amplifiers generally require about 8-10 dB of overhead input power ratio in order to maintain linearity in the RF waveforms.
The transmitter power amplifier consumes a constant and comparatively large amount of power, regardless of the relative strength of the output signal transmitted by the base transceiver station. For example, if the normal traffic load during the daytime requires the RF output power level to be approximately 10 watts, the DC prime power consumed by the transmitter power amplifier is approximately 80-100 watts (i.e., 8-10 dB higher). However, in the middle of the night, when the traffic load is very light, the RF output power level of the transmitter may be reduced in decrements down to, for example, about 1 watt, as power control is exercised over the RF output signal as described above. However, the DC prime power consumed by the transmitter power amplifier will still be approximately 80-100 watts, since the operating bias points of the power amplifiers are fixed. In the prior art systems, no consideration is given to whether the DC power consumption of the transmitter amplifier can be reduced and still maintain the existing output RF power level.
There is therefore a need in the art for improved cellular systems that are less expensive to operate. In particular, there is a need in the art for improved CDMA cellular systems that implement power control in the power amplifiers of the base station transmitters. Improved systems are needed that monitor and maintain an RF output signal level of an amplifier, while simultaneously reducing the DC power level required to produce the RF output signal.